View Full Version : Swapping bolt carrier on SR-15....sacrilege?
lennyo3034
06-08-16, 09:59
I am considering swapping out the bolt carrier on my SR-15 for a coated carrier. I notice my coated carriers (NiB, chrome, QPQ) on other rifles run and cycle a lot smoother than the basic phosphate on the KAC carrier.
Has anyone else tried this? With a different rifle, I wouldn't hesitate but I'm just not sure I want to start swapping out operating components on the KAC system which is supposed to be finely tuned.
Before anyone goes and mentions the e3 bolt and such, I am not replacing the entire BCG. Only the carrier.
Smoother? What does that even mean? Is the rifle running as advertised? Id absolutely leave it.
As my granddad used to say, "you dun lost yore dam mind".'
:)
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wildcard600
06-08-16, 11:32
Add some more lube.
What problem would you be solving by using a different carrier?
Lost River
06-08-16, 12:06
Unless you are experiencing malfunctions, don't screw with it.
You might find this hard to believe, but the people at KAC actually know how to build rifles.
I have spent more than a bit of time at various defense manufacturers like Knight's Armament, FN, Colt Defense, etc. and I can say for a fact that they know what they are doing.
When hobbyists (who have been successfully advertised to) start tinkering with stuff, they quite often end up with a less reliable platform.
I think the KAC carrier is just a standard mil spec unit so swapping with another quality carrier shouldn't cause any problems but I have to question the need.
Eurodriver
06-08-16, 12:32
I am considering swapping out the bolt carrier on my SR-15 for a coated carrier. I notice my coated carriers (NiB, chrome, QPQ) on other rifles run and cycle a lot smoother than the basic phosphate on the KAC carrier.
How many rounds do you have through your KAC?
lennyo3034
06-08-16, 12:52
The rifle in question just doesn't seem to shoot as well as my other SR-15 mod 1, with the exact same configuration. The recoil impulse feels smoother and lighter on my other one. I'm looking at the machining and the one that seems harsher has a very poorly machined carrier. Which is why I'm pointing at the carrier as the cause.
Round count could also be a factor. The one that shoots smoothly has about 12k rounds down it. The one I'm concerned about only has about 3k. However 3k rounds should be plenty to break in a carrier.
lennyo3034
06-08-16, 12:55
Not questioning KAC at all, but there are always improvements. KAC themselves have even said there are better finishes for the carrier than standard phosphate. I could be wrong, but I thought SR-16 carbon cutter carriers had an exterior finish?
Unless you are experiencing malfunctions, don't screw with it.
You might find this hard to believe, but the people at KAC actually know how to build rifles.
I have spent more than a bit of time at various defense manufacturers like Knight's Armament, FN, Colt Defense, etc. and I can say for a fact that they know what they are doing.
When hobbyists (who have been successfully advertised to) start tinkering with stuff, they quite often end up with a less reliable platform.
Random question: do you live in WV? I happen to know the lost river area well.
vereceleritas
06-08-16, 13:48
The rifle in question just doesn't seem to shoot as well as my other SR-15 mod 1, with the exact same configuration. The recoil impulse feels smoother and lighter on my other one. I'm looking at the machining and the one that seems harsher has a very poorly machined carrier. Which is why I'm pointing at the carrier as the cause.
If a harsher recoil impulse is the only symptom, it sounds like an oversized gas port is the issue. If that is the case, a different carrier coating is not going to help. You should consider trying a heavier buffer or an adjustable gas block. An LMT enhanced carrier may help but that has nothing to do with a fancy coating.
Anecdotally, it seems KAC has increased the size of their gas ports over time to accommodate weak ammo. My buddy's Mod 0 is noticeably softer shooting than the Mod 1 that I had.
All that being said, if it's functioning reliably as is, I wouldn't bother changing anything.
masenomics
06-08-16, 13:57
Sure replace it and send me the KAC carrier.
lennyo3034
06-08-16, 14:23
If a harsher recoil impulse is the only symptom, it sounds like an oversized gas port is the issue. If that is the case, a different carrier coating is not going to help. You should consider trying a heavier buffer or an adjustable gas block. An LMT enhanced carrier may help but that has nothing to do with a fancy coating.
Anecdotally, it seems KAC has increased the size of their gas ports over time to accommodate weak ammo. My buddy's Mod 0 is noticeably softer shooting than the Mod 1 that I had.
All that being said, if it's functioning reliably as is, I wouldn't bother changing anything.
I think you're right. Unfortunately an oversized port is much harder to fix than a bad carrier. I can definitely feel the difference when hand cycling the two rifles, however I'm not sure how much that matters when firing. Different sized ports should have much more of an effect.
The LMT enhanced carrier might be overkill. It's still softer shooting than most rifles out there, just not quite as soft as my other KAC.
I happen to have a Sionics np3 carrier lying around (I don't remember ordering it, no idea where I got a NIW Sionics carrier) that I may try out anyways. I don't see how it can do any harm.
Do it, if it makes you happy. I think it will smooth out with proper lubrication and shooting.
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My first thought is, no doubt, leave the KAC alone and shoot it as is, particularly since its functioning properly. However, it is kinda strange that, post 3k rounds, the BCG still drags through the upper in the manner in which you describe. I run Colt M16 BCGs in every upper i own. in other words, i don't fiddle with coatings anymore. Ive had 100% reliability out of the Colt BCGs and i won't be changing them out. Some BCGs have dropped in and run slick as glass, others did grind through the upper for a minute. However, with proper lubrication, they all smoothed out over time...as in, a few hundred rounds and manually cycling the weapon etc. When you draw the charging handle/BCG back, where in the cycle does it encounter the most friction? Sometimes this is a canted receiver extension issue.
Im still tempted to say keep it KAC and shoot on. I can understand being inquisitive though.
Failure2Stop
06-08-16, 16:01
The carrier is a standard mil-spec carrier as found in rifles like Colt and FN.
While there are "better" exterior coatings on the market, virtually zero increase in performance is found with them, primarily because the outside of the carrier only contacts the upper receiver on the raised skids, and those are usually pretty smooth. Regardless, carbon doesn't tend to build up on the raised contact points or inside the upper on those paths. What chokes rifles is carbon built up inside the carrier mouth and dry gas rings.
So, while park isn't the best exterior process, it's good enough for the application. The interior chrome is pretty good, though there are some newer coatings that might make the system require less lubricant, but it can't repel the lubricant or it'll just blow away with the first shot.
If the supplied carrier isn't meeting expectation, we'll bring it in and make sure it isn't a problem that slipped past QC.
Lost River
06-08-16, 16:06
Not questioning KAC at all, but there are always improvements. KAC themselves have even said there are better finishes for the carrier than standard phosphate. I could be wrong, but I thought SR-16 carbon cutter carriers had an exterior finish?
Random question: do you live in WV? I happen to know the lost river area well.
"Lost River" refers to the Lost River mountain range in Idaho.
While I have spent some time in WV, doing train ups for DOS contracts, I am not from the area.
I am considering swapping out the bolt carrier on my SR-15 for a coated carrier. I notice my coated carriers (NiB, chrome, QPQ) on other rifles run and cycle a lot smoother than the basic phosphate on the KAC carrier.
Has anyone else tried this? With a different rifle, I wouldn't hesitate but I'm just not sure I want to start swapping out operating components on the KAC system which is supposed to be finely tuned.
Before anyone goes and mentions the e3 bolt and such, I am not replacing the entire BCG. Only the carrier.
Isn't the SR-15 cam pin diameter different (smaller) from a mil-spec? If that's so, wouldn't it be incompatible with a mil-spec bolt carrier's cam pin cutout?
the cam pin fits directly into the bolt, not the carrier. So long as the cam pin isn't enlarged in critical areas, and the KAC cam pin isn't, the cam pin size isn't an issue. To be more specific, the KAC cam pin is smaller where the cam pin slides into the E3 bolt (in order to increase material in a known weak point in the mil spec bolt), but the KAC cam pin expands out to mil spec dimensions in the area that rides in the carrier. KAC = Proprietary bolt and other small parts + mil spec M16 carrier.
lennyo3034
06-09-16, 07:02
The carrier is a standard mil-spec carrier as found in rifles like Colt and FN.
While there are "better" exterior coatings on the market, virtually zero increase in performance is found with them, primarily because the outside of the carrier only contacts the upper receiver on the raised skids, and those are usually pretty smooth. Regardless, carbon doesn't tend to build up on the raised contact points or inside the upper on those paths. What chokes rifles is carbon built up inside the carrier mouth and dry gas rings.
So, while park isn't the best exterior process, it's good enough for the application. The interior chrome is pretty good, though there are some newer coatings that might make the system require less lubricant, but it can't repel the lubricant or it'll just blow away with the first shot.
If the supplied carrier isn't meeting expectation, we'll bring it in and make sure it isn't a problem that slipped past QC.
I appreciate the offer, however I don't think it's necessary. I made the thread as more of a curiosity as it's certainly not a big enough deal to send back. I think it may be the edges of the rail that are causing this. While they are no longer sharp, the edges feel like they have a couple dings.
I did a little research and the differences between the two rifles may just be due to the build date. The softer shooting one is older, and may have been built before the change in gas port size was made.
I keep the carrier lubed up pretty good. Usually dripping when placed in the upper.
I threw the Sionics carrier in last night and messed with it. It was rough in the upper as well, but that's expected until I get some rounds through it.
I have switched to a Young Man. NM carrier.
It has things about it I think makes it better (More bearing surface, chromed, better tolerances). Whether it is or not is up to you.
I threw the Sionics carrier in last night and messed with it. It was rough in the upper as well, but that's expected until I get some rounds through it.
Hmmm. I don't think that's expected at all.
Eurodriver
06-09-16, 13:10
I have switched to a Young Man. NM carrier.
It has things about it I think makes it better (More bearing surface, chromed, better tolerances). Whether it is or not is up to you.
What makes you think those things make it better? And where is your source for "better tolerances" and what does that mean?
I have switched to a Young Man. NM carrier.
It has things about it I think makes it better (More bearing surface, chromed, better tolerances). Whether it is or not is up to you.
How would "more bearing surface" make it better as a BCG? Increasing the bearing surface would increase wear and drag over a standard BCG.
Like I said, you decide. If you're a "mil spec is the best, just buy a Colt" type, it won't appeal to you. if you believe every little bit counts, and like to tinker it strikes me as a good choice and I have enjoyed my results.
Here is from Rainiers web site:
"National Match Bolt Carrier by Young Manufacturing, Inc. This bolt carrier is 1/2 oz heavier and has more surface area at the carrier front. These additions provide improved alignment of the bolt carrier as it moves in the receiver, tighter tolerances, more consistent bolt-lockup, and generally much smoother operation. This bolt carrier causes less wear, improved accuracy, and overall better function. All Y/M carriers are made using these high standards. The extra precision, quality, and post production processes are among the reasons top custom companies choose Young Manufacturing, Inc bolt carriers. You should too! (Available in AR-15 and M-16)
Features:
1⁄2 oz heavier than standard carrier
Full diameter at carrier front provides increased support and alignment strength in battery position
Carriers precision ground between centers after heat treatment
Removes distortions from heat treat process Ensures straightness and accuracy
Full hammer ramp machined for smoother operation
Configuration:
SEMI
Full Auto
***Properly Staked by Rainier Arms***"
https://www.rainierarms.com/young-mfg-national-match-bolt-carrier-group
How would "more bearing surface" make it better as a BCG? Increasing the bearing surface would increase wear and drag over a standard BCG.
More surface spreads the drag, and pressure out over more area.
Taken from JP enterprises web site, again about more bearing surface, similar idea to Young NM.
"Designed with extended forward assist serrations and a dust cover notch, our FMOS™ carriers also feature a 100% increase in bearing surface for smoother operation, improved alignment and lower wear in the upper receiver as well as a longer Mil-spec. cocking pad for optimal reliability"
http://jprifles.com/1.4.7_FMOS.php
OP, you're not going to hurt a damn thing by throwing in a quality coated carrier in there. Personally, I'd use the money for ammo and see if it smooths out, but if you have the disposable income, go for it. It will probably feel smoother to you- at least hand cycling it, and be slightly easier to clean. KAC makes a good tough product, it will tolerate a carrier swap just fine.
If you want to keep it KAC, why not send the carrier out and get it NP3 coated, or something similar. It might not be the flavor for everyone, but it is an option if you wanted to play around.
You do not want the carrier to support the bolt during firing. You want the bolt to be supported by the case pressure and the BE. An influence by the carrier at this point induces drag that you do not want.
Adding bearing surface around the circumference of a carrier is a net negative. Axial or longitudinal support may help higher stressed areas.
Even though it seems that other carriers may just drop in, it's not that simple. Adding a LMT enhanced carrier may seem like an easy addition, but you may find that the contact areas of the carrier to BE are not on the flat parallel surfaces, but on the extension on the carrier to the taper on the BE lugs, not ideal. You can easily resolve that issue, but it is an issue to resolve.
As far as coatings go, the standard finish may not be the best, but it is sacrificial and tends to hold lube. A surface imperfection can be masked by this. Other harder coatings are not as sacrificial, meaning that surface imperfections are not masked and can wear other components. I personally like the park and hard chrome, but hard chrome does take much more parts prep. Hard chrome can be scrubbed with a bronze brush that would wear a parked component. Bronze brushes clean faster than plastic ones.
As far as recoil/cycling characteristics, the OP may be better off by simply swapping in a heavier buffer to compensate for an upper that may be gassed harder than the other. I have many KAC rifles, non of my MOD 1 SR-15 rifles have the 3.0 ounce buffer anymore, early pre MOD 1 rifles run well with it. I've switched to mostly A5 variants, but I still have some carbine ones with heavier buffers and a few early guns with the 3.0 ounce buffer.
Buffer swaps are pretty easy, I would tend to steer the OP in that direction if the carrier does not need to be sent back for an issue first.
patriot_man
06-11-16, 06:50
When I had my SR-15 I used a chromed carrier as well.
I thought the SR-16 has one - might as well throw this extra carrier I have in there. But I didn't have the cool sand cuts lol
I have switched to a Young Man. NM carrier.
It has things about it I think makes it better (More bearing surface, chromed, better tolerances). Whether it is or not is up to you.
What accuracy gains did you realize when you installed the Young Manufacturing NM carrier? What else makes it better?
I've seen ARs with GI BCGs that shot 1/2 MOA off the bench with good reloads and remain reliable for thousands of rounds. I'm all for a better mousetrap, but what is gained by switching to the Young Manufacturing carrier?
Anything on a GI carrier or commercial equivalent that makes it "feel" rough usually smooths out with lubrication and use.
What accuracy gains did you realize when you installed the Young Manufacturing NM carrier? What else makes it better?
I've seen ARs with GI BCGs that shot 1/2 MOA off the bench with good reloads and remain reliable for thousands of rounds. I'm all for a better mousetrap, but what is gained by switching to the Young Manufacturing carrier?
Anything on a GI carrier or commercial equivalent that makes it "feel" rough usually smooths out with lubrication and use.
That is never a claim I made. That is rainiers/ young manufactorings claim.
Maybe I did pick up 1/10th of an MOA, no clue, I'd never peronally be able to tell. But if it is a potential it may be worth it. On my noveske barreled SPR that runs a young bcg this would be beneficial. Is it easy to quantify? No. There for I'll never make that claim. I did provide claims made by the manufacturer.
I did say I think tolerances are better. Being center ground after heat treating. I certainly like the chrome and the easy clean up it provides.
Tom says more surface is not better, he seems smart and an engineer. Sadly he never bothers to post in a language most of us would ever understand. He posts as if he was speaking to another fire arms engineer. This makes his most likely very informative posts almost useless to me. What I do know is the same pressure over more surface area distributes that pressure lessening its pressure per square inch.
When buying a standard bcg of quality. Vs a young NM the price difference isn't huge. What I've personally gained it was worth the 45 dollars extra to me.
Some one like Molon would be better suited to determine if it adds accuracy.
Send your KAC carrier to Robar and get it NP3+ coated, if you want a coating.
I would also try a heavier buffer as Tom 12 suggests above.
That is never a claim I made. That is rainiers/ young manufactorings claim.
Maybe I did pick up 1/10th of an MOA, no clue, I'd never peronally be able to tell. But if it is a potential it may be worth it. On my noveske barreled SPR that runs a young bcg this would be beneficial. Is it easy to quantify? No. There for I'll never make that claim. I did provide claims made by the manufacturer.
I did say I think tolerances are better. Being center ground after heat treating. I certainly like the chrome and the easy clean up it provides.
Tom says more surface is not better, he seems smart and an engineer. Sadly he never bothers to post in a language most of us would ever understand. He posts as if he was speaking to another fire arms engineer. This makes his most likely very informative posts almost useless to me. What I do know is the same pressure over more surface area distributes that pressure lessening its pressure per square inch.
When buying a standard bcg of quality. Vs a young NM the price difference isn't huge. What I've personally gained it was worth the 45 dollars extra to me.
Some one like Molon would be better suited to determine if it adds accuracy.
Roger that. Molon would be a good candidate for testing the product. He is very detailed in his testing.
I believe even pressure exerted by the bolt on the brass and even pressure between the bolt and the barrel extension are critical points for consistent accuracy.
As far as durability is concerned, time will tell.
"Smoothness" is not a critical issue for me personally as long as a weapon is durable and accurate. In most instances I can't tell, because when a task requires the hands of a surgeon, I have the hands of a blacksmith.
I don't have an issue with someone wanting to purchase the latest bling for their AR. It is more cost effective for someone like myself to spend the money on targets, ammunition and range time and if something works, I am reluctant to replace it.
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